1. Field of the Invention
The present invention relates to an image coding apparatus and a method thereof and an image decoding apparatus and a method thereof, which use wavelet conversion to encode a still image or a motion picture.
2. Description of the Related Art
A conventional representative image compression method is a JPEG (Joint Photographic Coding Experts Group) system standardized by ISO (International Organization for Standardization). In this JPEG system, DCT (Discrete Cosine Transform) is used to compress and encode mainly still images. It is known that this system provides excellent coded/decoded images if a relatively high bit number is assigned. In this system, however, block deformation specific to DCT becomes conspicuous so that deterioration subjectively becomes conspicuous.
Differently from the above system, studies have recently been made of a system in which an image is divided into a plurality of band ranges called filter banks by a filter which combines a high-pass filter and a low pass filter, and coding is carried out for every band range. Particularly, wavelet coding is strongly regarded as a new technique which will substitute the existing DCT technique because the wavelet coding excludes the drawback of the DCT technique, i.e., block deformation becomes conspicuous under high compression.
Meanwhile, MPEG (Motion Picture Experts Group) system is used for motion picture coding. MPEG-1, MPEG-2, and MPEG-4 are known at present. Particularly, the MPEG-2 is widely used for video compression and the like for DVD (Digital Versatile Disc). In a coding means for the JPEG and MPEG systems, coding control is performed for every macro block (normal: 16×16) constructed by 8×8 blocks as processing units of the DCT.
Presently, many products such as electronic still cameras, video movies, and the like adopt the JPEG system, the MPEG system, or a so-called DV (Digital Video) system. Each of these compression coding systems adopts DCT for its conversion system. It is supposed that products as described above, which are based on wavelet conversion, will appear on the market in the future. Discussions for improvements in the efficiency of the coding system are eagerly carried out by research organizations. Actually, JPEG 2000 (being prepared by ISO/IEC/JTC1SC29/WG1 which is the same organization as JPEG), which is expected to be the international standard system for still images and which can be said to be a follower in the next generation, is a format from which a standardization recommendation Part-1 is to be issued in December 2000. According to the JPEG 2000, it has been decided to adopt wavelet conversion in place of existing DCT of the JPEG, as a conversion system as the basis of image compression.
In order to obtain coded images with high quality with respect to not only still images but also motion pictures by means of the wavelet conversion, it is important to solve problems as will be described below.
(1) At FCD (Final Committee Draft) concerning Part-1 of JPEG-2000, there are two filters for wavelet conversion which is presently defined in July 2000. One is a 5×3 filter of an Integer type for reversible conversion, and another filter is a 7×9 filter of a Float type for irreversible conversion.
(2) Compared with the Integer type 5×3 filter, the Float type 7×9 filter has much higher structural complexity, so there is a problem in using filters of both types to construct hardware. In addition, in order to guarantee the precision of the floating point in the latter type, a dedicated floating point calculator is required, resulting in a problem that the circuit scale of the hardware must be enlarged.
(3) On the other side, as a result of experiments, a 5×3 filter of a fixed-point type which is obtained as a more precise version of the 5×3 filter of the Integer type described not only achieves a coding efficiency of excellent performance equivalent to that of the 7×9 filter of the Float type as described above but also has parts common to the 5×3 filter of the Integer type, in its internal calculators. Accordingly, there is an advantage in that enlargement of hardware can be reduced to the least by comprising both filters, without sacrificing the coding efficiency.
(4) FCD according to Part-1 of JPEG-2000 describes a calculation expression for the 5×3 filter of the Integer type as described above. In accordance with the procedure thereof, a wavelet conversion coefficient can be generated. However, the FCD includes no description about the calculation means of the 5×3 filter of a Fixed-point type. Settlement of compatibility between both filters relates to settlement of a common circuit as described above and is thus very important.